Image Forming Apparatus - Patent 8103184

This application is based on application No. 2008-160468 filed in Japan, the entire content of which is hereby incorporated by reference.TECHNICAL FIELD The present invention relates to an image forming apparatus with a contact transfer system which passes transfer paper through a contact section between an image carrier and a transfer roller so as to transfer a toner image onto the transferpaper.BACKGROUND ART There have been copying machines, facsimiles, printers and the like as examples of an image forming apparatus which uses electro photography process, and which passes transfer paper through a contact section between an image carrier and atransfer roller so as to transfer a toner image onto the transfer paper. In these image forming apparatuses, when the transfer roller is cleaned, the transfer roller is brought into contact with the image carrier. Thereafter, a positive polaritypotential and a negative polarity potential are alternately applied to the transfer roller so as to transfer the toner attached to the transfer roller onto the image carrier by repulsion. However, there have been a problem that the toner attached to the transfer roller is not fully cleaned in this electric cleaning method, so that contamination is generated in the top end and the back end of transfer materials. Accordingly, amethod has been proposed, in which a circumferential speed of the image carrier and the transfer roller is changed between during transfer operation and during cleaning operation, as in an image forming apparatus disclosed in JP 08-272233 A. FIG. 10 shows parts including the image carrier and the transfer roller in the conventional image forming apparatus disclosed in JP 08-272233 A. In FIG. 10, during cleaning operation of the transfer roller 1, a photoconductor 2 is charged to anegative polarity by the charging roller 3, so that optical write by an exposure means 4 is not performed. Also, a developing roller of a developing device 5 is not driven, and transf